The present invention relates to wave soldering apparatus and more particularly, to an durable solder reservoir and a nozzle/pump assembly which withstand high temperatures and which are less subject to wear due to rubbing contact with solder within the solder reservoir.
A usual wave soldering apparatus includes conveyors driven to advance printed circuit boards at a constant speed from the entrance to exit ends of the apparatus. The board is first carried to a fluxer where flux is applied to the underside of the board. The printed circuit board is then carried over preheaters where the temperature of the board is elevated to approximately 110° C. to 130° C. so as to evaporate excess flux solvent, activate the flux and minimize thermal shock to the printed circuit board. After the printed circuit board is brought to such a preheat temperature, the board passes over a solder reservoir. A plurality of electric heaters are arranged within the solder reservoir to maintain solder in a molten state. At least one solder wave nozzle is disposed in the solder reservoir and connected to a pump. The pump forces the molten solder up through the nozzle to form a solder wave. Electronic components in the board are contacted with the solder wave to make solder joints.
Typically, the heaters, the nozzle and the pump are made of stainless steel and each covered with a chromium oxide layer. The chromium oxide layer effectively retards adherence of the solder to the heaters, the nozzle and the pump. However, the chromium oxide layer is likely to be removed from the heaters since they are exposed to high temperatures. Also, the chromium oxide layer is likely to be removed from part of the nozzle and the pump due to rubbing contact with the solder. In the latter case, the molten solder acts as an abrasive. If the chromium oxide layer is damaged or removed, iron and nickel contained in the stainless steel become alloyed with tin contained in the solder. The resulting alloys melt into the molten solder within the solder reservoir. This results in damage to the heaters, the nozzle and the pump.
Japanese laid-open patent publication No. 61-82966 discloses that a solder wave nozzle is coated with Teflon. However, such a Teflon coating is easily damaged or removed at high temperatures. Also, impingement of solder against the nozzle accelerates removal of the Teflon coating from the nozzle. Japanese laid-open patent publication No. 11-47918 teaches the use of a compound of sulfide and nitride. Such a compound effectively prevents adherence of solder to a solder reservoir, but it is rather expensive and cumbersome to form the compound on the surface of the solder reservoir.
Accordingly, it is an object of the present invention to provide a wave soldering apparatus which effectively and economically prevents adherence of solder to heaters, nozzles and other members of the apparatus regardless of whether they are exposed to high temperatures or subjected to rubbing contact with the solder.